Is clean my mac classic an antivirus?

NO. CleanMyMac is NOT an anti-virus application. But it's about as close to outright malware as you can get.

MacOS is very secure and quite capable of keeping itself that way. There is no need to run any “cleaning”, “speed-up” or “optimizing” apps, nor any anti-virus apps.

In fact, most anti-virus and cleaning apps are less than helpful on the Mac and often cause more problems and headache than they prevent because they conflict with the Mac’s built in securities.

Please see these Apple Support Community documents:

click> Effective Defenses Against Malware - Apple Community

click> How to install adware - Apple Community

click> Avoid phishing emails, fake 'virus' alerts…and other scams - Apple Support 

click> macOS security built right in. - Apple 

If you currently have this type of app installed, you should uninstall it according to the developer's instructions, and continue to use best practices when using your Mac.

If you have been advised as a  troubleshooting measure to do a malware scan on your Mac, then Malwarebytes is a safe and recommended utility. And the free version is all you need to get started.

ClamAV is presently the only malware removal that is recommend for Snow Leopard.

Malwarebytes won't run on Snow Leopard.

If you are hanging on to Snow Leopard because:

1. you are not running a Core2Duo or later processor (Xeon, i3, i,5, i7 are later).
2. you have a PowerPC only application, read this tip: https://discussions.apple.com/docs/DOC-6271
3. you don't need any of the features of newer systems.

Keep in mind security by obscurity does work. Chances are you will not get infected. You might find your system slow down because of age, the storage getting too full, or using optimizers that actually slow you down.

There is an Etrecheck for Snow Leopard if you want to find out if you need help removing a background application that is slowing you down.

Contact the author to get a copy.